1. Field of the Invention
This invention relates to a shearing interferometer for measuring lens lateral aberration, specifically to an interferometer which uses the shearing interferometer method for measuring the lateral aberration of a lens.
2. Prior Art Statement
The method of obtaining the lateral aberration curve of a lens using shearing interference fringes has conventionally required analysis of differential interference fringes, making it complicated and time consuming. The inventors studied the possibility of developing a method for enabling the aberration curve to be represented by the shearing interference fringes themselves.
As set out in Japanese Patent Application No. Hei 3-89600, the inventors earlier developed a shearing interferometer for obtaining the lateral aberration curve of a lens, which obtains an interference fringe corresponding to the lateral aberration curve by using a slightly wedged plate to impart horizontal shear to an incident wavefront and then imparting a tilt to the slightly wedged plate by rotating it slightly about its horizontal axis. Their analysis to date shows that when a shear of .DELTA.S is imparted in the x direction and a tilt of .alpha. is imparted in the y direction, the lateral aberration .phi. is represented by EQU .phi.=f0(.lambda./.DELTA.S) (.DELTA.m(x:.DELTA.S)-.alpha.y)
where f0 is the focal length of the tested lens, .lambda. is the wavelength of the light source used, and .DELTA.m is the order of the interference fringes obtained.
Although the sensitivity for a given amount of aberration and shear can be increased by reducing the amount of tilt .alpha., simply doing this causes the interference fringes to spread to such a great extent that it becomes difficult to ascertain accurately the center of the interference fringes.